Heretofore, as shown in FIG. 12, a resonant actuator has been known that includes a ceramic base member 101 formed to have a square bar shape and electrodes 102a and 102b provided on two end surfaces thereof.
This resonant actuator is polarized in the direction shown by arrow a and is driven at a resonant frequency or at a frequency in the vicinity thereof by applying an alternating electric field to the electrodes 102a and 102b so as to obtain a longitudinal oscillation in the direction of arrow b, which is the same direction as the arrow a direction. In general, the electrodes 102a and 102b and lead wires 104a and 104b are held by spring terminals 103a and 103b, respectively, or the electrodes 102a and 102b are directly connected to lead wires 104a and 104b, respectively, and an alternating electric field is applied to the electrodes 102a and 102b. 
In this type of resonant actuator, it has been believed that the amount of displacement in the oscillation direction is generally proportional to the piezoelectric constant d. Accordingly, as a ceramic material for resonant actuators, research and development of a piezoelectric material primarily including Pb(Zr,Ti)O3 (lead zirconate titanate; hereinafter referred to as “PZT”) that has a large piezoelectric constant has heretofore been aggressively carried out.
Since power devices, such as a piezoelectric actuator, use the large-amplitude elastic oscillation of a piezoelectric ceramic, large amplitude characteristics of piezoelectric ceramics have been disclosed for example, in Non-Patent Document 1.
In this Non-Patent Document 1, it has been reported that although the oscillation speed (equal to oscillation amplitude×frequency) is theoretically changed in proportion to an applied electric field E, when a PZT-based piezoelectric ceramic is driven at a resonant frequency, and when the electric field strength exceeds a certain predetermined level, the oscillation speed is gradually decreased to less than the theoretical value and is finally saturated. In addition, the relationship between an oscillation speed limit of PZT and a driving electric field has also been disclosed in this Non-Patent Document 1, and it has been reported that although the above oscillation speed limit changes with a change in material composition, the oscillation speed of a PZT-based piezoelectric ceramic material does not exceed 1 m/s at the maximum.
Since a high-power material having a high oscillation level has been desired in the fields of a piezoelectric actuator and the like, a method for evaluating piezoelectric properties, and the relationship between the composition of a PZT-based piezoelectric ceramic and high-power characteristics, such as oscillation level characteristics, have been reported in Non-Patent Document 2.
In this Non-Patent Document 2, it has been reported that when a PZT-based piezoelectric ceramic is driven at a resonant frequency, and when the oscillation level exceeds a certain predetermined level, the resonant frequency fr and the mechanical quality factor Qm are degraded.
[Non-Patent Document 1]: Sadayuki Takahashi, “New Development of Piezoelectric Materials”, TIC Corp., New Ceramics, Vol. 11, No. 8, (1988), pp. 29 to 34
[Non-Patent Document 2]: Sadayuki Takahashi, “Evaluation of High-Power Material”, TIC Corp., New Ceramics, (1995), No. 6, pp. 17 to 21